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When he connected wires to his battery and a piece of carbon, the carbon glowed, producing light. His invention was known as the Electric Arc lamp. More notably, in , British scientist Warren de la Rue enclosed a coiled platinum filament in a vacuum tube and passed an electric current through it. The design was based on the concept that the high melting point of platinum would allow it to operate at high temperatures and that the evacuated chamber would contain fewer gas molecules to react with the platinum, improving its longevity. Although an efficient design, the cost of the platinum made it impractical for commercial production.

And by he had a working prototype, but the lack of a good vacuum and an adequate supply of electricity resulted in a bulb whose lifetime was much too short to be considered an effective prodcer of light. In , Swan developed a longer lasting light bulb using a treated cotton thread that also removed the problem of early bulb blackening.

They built their lamps with different sizes and shapes of carbon rods held between electrodes in glass cylinders filled with nitrogen. Woodward and Evans attempted to commercialize their lamp, but were unsuccessful. They eventually sold their patent to Edison in In , Thomas Edison began serious research into developing a practical incandescent lamp and on October 14, , Edison filed his first patent application for "Improvement In Electric Lights".

However, he continued to test several types of material for metal filaments to improve upon his original design and by Nov 4, , he filed another U. Although the patent described several ways of creating the carbon filament including using "cotton and linen thread, wood splints, papers coiled in various ways," it was not until several months after the patent was granted that Edison and his team discovered that a carbonized bamboo filament could last over hours.

The remaining energy is lost as heat. However these inefficient light bulbs are still widely used today due to many advantages such as:. When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal or rather normal to the boundary, the change in wavelength results in a change in the direction of the beam.

This change of direction is known as refraction. The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses , spectacles , contact lenses , microscopes and refracting telescopes are all examples of this manipulation. There are many sources of light. A body at a given temperature emits a characteristic spectrum of black-body radiation.

A common thermal light source in history is the glowing solid particles in flames , but these also emit most of their radiation in the infrared, and only a fraction in the visible spectrum.


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The peak of the blackbody spectrum is in the deep infrared, at about 10 micrometre wavelength, for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue-white colour as the peak moves out of the visible part of the spectrum and into the ultraviolet.

These colours can be seen when metal is heated to "red hot" or "white hot". Atoms emit and absorb light at characteristic energies. This produces " emission lines " in the spectrum of each atom. Emission can be spontaneous , as in light-emitting diodes , gas discharge lamps such as neon lamps and neon signs , mercury-vapor lamps , etc. Emission can also be stimulated , as in a laser or a microwave maser.

Deceleration of a free charged particle, such as an electron , can produce visible radiation: Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation. Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake. Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation.

This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors. When the concept of light is intended to include very-high-energy photons gamma rays , additional generation mechanisms include:. Light is measured with two main alternative sets of units: Photometry is useful, for example, to quantify Illumination lighting intended for human use.

The SI units for both systems are summarised in the following tables. The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The photometry units are designed to take this into account, and therefore are a better representation of how "bright" a light appears to be than raw intensity.

They relate to raw power by a quantity called luminous efficacy , and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a photocell sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and charge-coupled devices CCD tend to respond to some infrared , ultraviolet or both.

Light exerts physical pressure on objects in its path, a phenomenon which can be deduced by Maxwell's equations, but can be more easily explained by the particle nature of light: Light pressure is equal to the power of the light beam divided by c , the speed of light. Due to the magnitude of c , the effect of light pressure is negligible for everyday objects.

For example, a one-milliwatt laser pointer exerts a force of about 3. The possibility of making solar sails that would accelerate spaceships in space is also under investigation. Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum. The forces of pressure exerted on the two sides are equal if the plate is at rest.

However, if it is in motion, more radiation will be reflected on the surface that is ahead during the motion front surface than on the back surface. The backwardacting force of pressure exerted on the front surface is thus larger than the force of pressure acting on the back. Hence, as the resultant of the two forces, there remains a force that counteracts the motion of the plate and that increases with the velocity of the plate. We will call this resultant 'radiation friction' in brief. In the fifth century BC, Empedocles postulated that everything was composed of four elements ; fire, air, earth and water.

History of the Light Bulb

He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.

In about BC, Euclid wrote Optica , in which he studied the properties of light.

Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. If the beam from the eye travels infinitely fast this is not a problem. Despite being similar to later particle theories, Lucretius's views were not generally accepted.

In ancient India , the Hindu schools of Samkhya and Vaisheshika , from around the early centuries AD developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements tanmatra out of which emerge the gross elements.

The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous. The basic atoms are those of earth prthivi , water pani , fire agni , and air vayu Light rays are taken to be a stream of high velocity of tejas fire atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms. They viewed light as being an atomic entity equivalent to energy. Descartes arrived at this conclusion by analogy with the behaviour of sound waves.

Descartes is not the first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes' theory of light is regarded as the start of modern physical optics. Pierre Gassendi — , an atomist, proposed a particle theory of light which was published posthumously in the s.

10th-11th centuries

Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of that light was composed of corpuscles particles of matter which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines.

He did, however, explain the phenomenon of the diffraction of light which had been observed by Francesco Grimaldi by allowing that a light particle could create a localised wave in the aether.


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Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of His reputation helped the particle theory of light to hold sway during the 18th century. The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words, it would become what is now called a black hole. Laplace withdrew his suggestion later, after a wave theory of light became firmly established as the model for light as has been explained, neither a particle or wave theory is fully correct.

A translation of Newton's essay on light appears in The large scale structure of space-time, by Stephen Hawking and George F. The fact that light could be polarized was for the first time qualitatively explained by Newton using the particle theory. Jean-Baptiste Biot in showed that this theory explained all known phenomena of light polarization. At that time the polarization was considered as the proof of the particle theory. To explain the origin of colors, Robert Hooke developed a "pulse theory" and compared the spreading of light to that of waves in water in his work Micrographia "Observation IX".

In Hooke suggested that light's vibrations could be perpendicular to the direction of propagation. Christiaan Huygens worked out a mathematical wave theory of light in , and published it in his Treatise on light in He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. The wave theory predicted that light waves could interfere with each other like sound waves as noted around by Thomas Young.

"Without light, no life"

Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye. Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum that diffraction could more easily be explained by a wave theory.

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. The existence of the hypothetical substance luminiferous aether proposed by Huygens in was cast into strong doubt in the late nineteenth century by the Michelson—Morley experiment.

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. In , Michael Faraday discovered that the plane of polarisation of linearly polarised light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric , an effect now known as Faraday rotation.

In he speculated that light might be some form of disturbance propagating along magnetic field lines. Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: In , he published A Treatise on Electricity and Magnetism , which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations.

Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.

In the quantum theory, photons are seen as wave packets of the waves described in the classical theory of Maxwell.

Light - Wikipedia

The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not such as spectral lines. In Max Planck , attempting to explain black body radiation suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. Planck called these "lumps" of light energy "quanta" from a Latin word for "how much".

In , Albert Einstein used the idea of light quanta to explain the photoelectric effect , and suggested that these light quanta had a "real" existence. In Arthur Holly Compton showed that the wavelength shift seen when low intensity X-rays scattered from electrons so called Compton scattering could be explained by a particle-theory of X-rays, but not a wave theory. In Gilbert N. Lewis named these light quanta particles photons. Eventually the modern theory of quantum mechanics came to picture light as in some sense both a particle and a wave, and in another sense , as a phenomenon which is neither a particle nor a wave which actually are macroscopic phenomena, such as baseballs or ocean waves.

Instead, modern physics sees light as something that can be described sometimes with mathematics appropriate to one type of macroscopic metaphor particles , and sometimes another macroscopic metaphor water waves , but is actually something that cannot be fully imagined. As in the case for radio waves and the X-rays involved in Compton scattering, physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other.

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Visible light, which occupies a middle ground in frequency, can easily be shown in experiments to be describable using either a wave or particle model, or sometimes both. In February , scientists reported, for the first time, the discovery of a new form of light, which may involve polaritons , that could be useful in the development of quantum computers. From Wikipedia, the free encyclopedia. For light that cannot be seen with human eye, see Electromagnetic radiation. For other uses, see Light disambiguation and Visible light disambiguation.

For the solar energy developer named Lightsource, see Lightsource Renewable Energy. List of light sources. Photometry optics and Radiometry. SI radiometry units v t e. SI photometry quantities v t e. This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. May Learn how and when to remove this template message. Corpuscular theory of light. Physics portal Science portal.

By the International Lighting Vocabulary , the definition of light is: Textbook of Practical Physiology 1st ed. Retrieved 11 October The human eye has the ability to respond to all the wavelengths of light from — nm. This is called the visible part of the spectrum. The Natural Laws of the Universe: Introduction to Optics and Lasers in Engineering. Retrieved 20 October A Physical Approach to Astronomical Observations. Handbook of Pharmaceutical Analysis.